The present invention relates, in general, to level detection apparatus.
Liquid level detection apparatus utilizing wave guides are widely employed in diverse applications to measure the level of a liquid or solid fluent material in enclosures, such as tanks. Such wave guide-based level detection apparatus operate on the principal of signal reflection caused by disparities between the top surface of the liquid or solid fluent material in the tank and the different liquid gas located above the top surface. Such wave guide-based apparatus can operate on radar and laser principles to reflect a signal off of the top surface of the material in a tank to determine the level of the material in the tank.
The phenomenon of magnetostriction has been widely employed in linear distance and position measuring devices. A magnet located near or around a magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet, an electrical signal is generated in the wire. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet, a torsional strain is generated in the wire.
Such linear position detectors using a magnet mounted in a float have been utilized as liquid level detectors to provide an indication of a level of liquid within a tank, such as an underground tank. The position of the magnet, and hence, the liquid level, is determined as a function of time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet in the case of mechanical excitation, or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
Other types of magnetostrictive position measuring devices utilize a reflective termination at the foot end of the magnetostrictive wire. Such devices measure the difference between the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and a pulse traveling on the wire directly from the magnet to the head.
As shown in U.S. Pat. Nos. 4,839,590; 5,017,867; 5,050,430; and 5,253,521, all of which are assigned to the assignee of the present invention, such magnetostrictive devices include an elongated, small diameter tube, typically made of stainless steel, on which a movable magnet is mounted for providing an indication of a liquid level. A head and cap are mounted on one end of the tube, typically above the liquid level, and enclose electronic components, such as terminal connections and a signal conditioning circuit used to supply or output signals to and from the magnetostrictive wire in the tube.
Some liquid level detection applications require probe lengths of more than 20 feet. Since such probes are typically constructed of a rigid brass or stainless steel tube, the length of the tube creates significant problems with respect to storage, shipping and installation of the probes. The long, straight probes make it difficult to install the probe in confined areas lacking sufficient clearance above the tank for insertion of the probe through a port in the tank. Additional problems are encountered when installing such long length probes in large above-ground tanks. Such tanks require the installer to carry the probe up a ladder on the exterior of the tank and then to insert the long probe through an opening in the top of the tank.
It is known to construct liquid level detection apparatus or probes with a flexible housing in the form of a magnetostrictive wire mounted within a flexible, plastic outer tube. However, the use of a flexible outer plastic tube does not meet the requirement of non-permeability to fluids. The use of plastic outer housings has resulted in the ingress of toxic, corrosive, and/or explosive fluids into the interior of the housing which interfere with the timed propagation of signals along the magnetostrictive wire.
The Assignee of the present invention previously devised a material level detection apparatus having a flexible outer housing as described in U.S. Pat. No. 5,929,763. The outer tube is formed of a semi-rigid flexible material which is non-permeable to the material in a tank, for example, yet is flexible enough to permit coiling to simplify installation, particularly in longer length level detection apparatus approaching fifty feet or more.
In order to maintain the flexible outer housing in as straight as possible a position in a tank for accurate level measurements, a weight was attached to the distal end of the outer housing as shown for the center level detection apparatus in FIG. 1. However, it was known that the typically plastic material used to form the outer housing has a significant coefficient of thermal expansion which can cause the overall length of the outer housing to vary many inches relative to the inner housing containing the magnetostrictive wire which is typically housed in a metal, such as a brass housing, inside of the outer, plastic housing. Due to the possibility of significant extension of the outer housing, the distal end of the outer housing had to be spaced a significant difference from the bottom of the tank to prevent contact with the tank which could lead to inaccurate level detection measurements due to bowing of the outer housing and the inner housing containing the magnetostrictive wire during extreme thermal expansion conditions.
Thermal expansion is not a major consideration for rigid magnetostrictive-based wave guide level detectors, as shown by the left most level detector in FIG. 1. However, since the bottom end of the rigid housing needs to be placed in contact with the bottom of the tank, accurate and somewhat costly tank height measurement must be taken in the field in the case of retrofit applications.
While the rigid housing application shown on the left in FIG. 1 places the bottom end of the wave guide or magnetostrictive wire extremely close to the bottom of the tank so as to provide a minimal dead zone or non-measurement zone at the bottom of the tank, such is not the case for the flexible outer housing shown in the center application in FIG. 1. As described above, due to the need to maintain the distal end of the flexible plastic outer housing and weight above the bottom of the tank at extreme thermal expansion conditions, the dead zone or non-measurement height in the bottom of the tank is increased from the rigid housing described above.
It would be desirable to provide a wave guide-based level detection apparatus which utilizes a flexible outer housing for ease of installation and at the same time has a minimal dead zone or non-measurement area in the bottom of the tank for a greater measurement range. It would also be desirable to provide such a wave guide-based level detection apparatus which can minimize the dead zone band of non-measurement in the bottom of the tank while simplifying wave guide installation by eliminating the need to accurately know or determine the inside height dimension of the tank.
The present invention is a level detection apparatus for detecting the surface level of material in an enclosure, such as a tank or reservoir.
In one aspect, the level detection apparatus includes a wave guide means having first and second opposed ends. A biasing means is fixed on an enclosure and exerts a biasing force on the wave guide means to accommodate thermal expansion. A weight is attached to the second end of the wave guide means for fixing the second end of the wave guide means on the bottom of the enclosure.
In another aspect, an outer flexible housing has first and second ends. The second end of an inner housing is fixed to the outer housing. A biasing means is coupled to the first end of the outer housing for exerting a biasing force on the housing to accommodate thermal expansion movement of the outer housing. Means are provided for fixedly mounting the biasing means relative to an enclosure. A weight is coupled to the second end of the outer tube to maintain the second end of the outer tube in close proximity to or directly on a bottom surface of the enclosure.
In one aspect, a sensor means is mounted to an inner housing having first and second opposed ends and an outer housing having first and second ends. The second end of the inner housing is fixed to the outer housing. Biasing means are coupled to the first end of the outer housing for exerting a biasing force on the outer housing to accommodate thermal expansion movement of the outer housing. Means are provided for fixedly mounting the biasing means. A weight is coupled to the second end of the outer tube.
In one aspect, the biasing means is a constant force spring having an end extending from a coil portion, the end attached to the outer housing.
In one exemplary use of the level detection apparatus of the present invention, the wave guide includes a magnetostrictive wire extending through the inner housing. The transducer means includes means for imparting a signal to the wire and for receiving a return signal propagated along the wire. The transducer means is preferably mounted on a circuit board fixed in position within the outer housing.
The support means in one aspect of the invention includes a coupling mountable on an enclosure cover and having a through bore for receiving a first end of the outer tube therethrough. A plurality of support rods are mounted in the coupling and extend outward from the coupling. Mounting means are provided for receiving opposite ends of the support rods. The mounting means carry a coiled portion of the biasing means. In a detailed aspect, the mounting means includes first and second bodies rotatably coupled to each other. The first body receives one end of the support rods, and the second body supports the biasing means.
The flexible level detection apparatus of the present invention provides numerous advantages in level detection apparatus, particularly detecting the top or surface level of material in a tank or enclosure. The apparatus employs a flexible housing which simplifies transportation to the installation site and actual installation of a wave guide in the housing in tank since the housing may be coiled prior to insertion into the tank. At the same time, the present apparatus accommodates thermal expansion and contraction of the flexible outer housing thereby creating a minimal dead zone near the bottom of the tank for accurate level measurements despite the varying position of the second end of the wave guide.